BACKGROUND OF THE INVENTION
[0001] RAIM is the abbreviation for Receiver Autonomous Integrity Monitoring, a technology
developed to assess the integrity of Global Positioning System (GPS) signals in a
GPS receiver system. It is of special importance in safety-critical GPS applications,
such as in aviation or marine navigation.
[0002] RAIM detects faults by utilizing redundant GPS pseudorange measurements. That is,
when more satellites are available than needed to produce a position fix, the extra
pseudoranges should all be consistent with the computed position. A pseudorange that
differs significantly from the expected value (i.e., an outlier) may indicate a fault
of the associated satellite or another signal integrity problem (e.g., ionospheric
dispersion). Traditional RAIM uses fault detection only (FD); however, newer GPS receivers
incorporate Fault Detection and Exclusion (FDE) which enables them to continue to
operate in the presence of a GPS failure.
[0003] Because RAIM operates autonomously, that is, without the assistance of external signals,
it requires redundant pseudorange measurements. To obtain a 3-dimensional position
solution, at least 4 measurements are required. To enable RAIM FD (Fault detection
in RAIM)" at least 5 measurements are required, and to enable RAIM FDE (Fault detection
in RAIM with the ability to exclude faulty data), at least 6 measurements are required.
However, more measurements are often needed depending on the satellite geometry. Typically,
there are 7 to 12 satellites in view.
[0004] Conventional RAIM availability thus requires 6 or more satellite measurements with
good satellite geometry. This is two or more satellites than is required for the basic
navigation solution. However, if time can be eliminated from the list of unknowns,
and thus drop the required number of satellites from 4 to 3, then RAIM FDE can be
achieved with only 5 satellites. Time can be eliminated by proving the GPS receiver
with a precise time reference such as that available from an atomic clock.
[0005] Since GPS requires "line of sight" reception to receive the GPS navigational signal,
terrain surrounding runways can occlude one or more of the satellites at critical
times. Removal of one or more of the several satellites compromises or prevents the
availability of RAIM. Aircraft GPS precision approaches are frequently interrupted
by RAIM outages. Certain flight operations, such as precision approach, can no longer
be executed without RAIM availability.
[0006] There is an unmet need in the art for improving the availability of RAIM by using
the aid a precise and accurate time signal.
SUMMARY OF THE INVENTION
[0007] A method and a system for providing a substituted timing signal for a missing satellite
ephemeris in execution of a RAIM algorithm includes deriving a plurality of position,
velocity, and time ("PVT") solutions from a GPS navigation system. The position, velocity
and time solutions are derived from a plurality of satellite pseudorange measurements
and ephemerides. An atomic clock provides an atomic clock signal. The atomic clock
signal is compared to the derived time solutions to arrive at a correction factor.
The atomic clock signal is adjusted according to the correction factor to develop
an adjusted atomic clock signal. The adjusted atomic clock signal can then be substituted
for a missing satellite measurement to execute the RAIM algorithm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] Preferred and alternative embodiments of the present invention are described in detail
below with reference to the following drawings:
[0009] FIGURE 1 is a block diagram of an exemplary GPS navigation system with an atomic
clock and clock follower; and
[0010] FIGURE 2 is a system of executing a RAIM algorithm based upon clock coasting with
an atomic clock.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0011] Receiver Autonomous Integrity Monitoring (RAIM) refers to a class of self-contained
GPS integrity monitoring methods based on a consistency check among redundant ranging
signals to detect an unacceptably large satellite range error due to either erroneous
satellite clock or erroneous satellite ephemeris data.
[0012] RAIM involves two types of functions. The first function is to detect whether a malfunction
that results in a large range error has occurred on any satellite, that is, to detect
the presence or absence of such a malfunction. The second function is to identify
the faulty ephemeris from a satellite from among the several ephemerides. Detection
requires at least 5 satellites be visible. Identification requires at least 6 by conventional
means.
[0013] FIG. 1 illustrates a navigation system 10, which includes a first atomic clock 18,
a second atomic clock 21, a GPS slaving unit 27, GPS receiver 30 and a system processor
33. In a non-limiting alternate embodiment, the navigation system includes a third
atomic clock 24. A clock processor 20 includes the GPS slaving unit 27 and is used
to slave the clocks 18, 21, 24 and to provide a frequency stable time standard to
the remainder of the navigation system 10.
[0014] The GPS receiver 30 is configured to receive navigation signals from GPS satellites.
The system processor 33 implements modified RAIM algorithms or functions enhanced
by the output from the GPS slaving unit 27. In this embodiment, the input from GPS
slaving unit 27 is used, not only to refine position, velocity, and time solutions,
but also to detect faults in the individual output of the plurality of atomic clocks
18, 21, 24 as may become prominent over a short time interval.
[0015] As in some prior art systems, in the navigation system 10 illustrated in FIG. 1,
the GPS satellite measurement provides a beginning time reference generally in a pulse
per second (PPS) interval signal. A PPS signal is an electrical signal that very precisely
indicates the start of a second. PPS signals are output by various types of precision
clock, including some models of GPS receivers. Depending on the source, properly operating
PPS signals have an accuracy ranging from a few nanoseconds to a few milliseconds.
[0016] PPS signals are used for precise timekeeping and time measurement. One increasingly
common use is in navigation system timekeeping, including the NTP protocol, which
is used to link the several subsystems in aircraft avionics. It should be noted that
because the PPS signal does not specify the time but merely the start of a second,
one must combine the PPS functionality with another time source that provides the
full date and time in order to ascertain the time both accurately and precisely. Nonetheless,
PPS signals can be extremely useful in slaving a plurality of clocks; in the case
of this invention, the atomic clocks 18, 21, 24.
[0017] The basic physics of atomic clocks 18, 21, 24 have been fairly well understood for
some time, along with the macro-engineering challenges in creating a clock 30 with
frequency stability of one part in 10 billion-equivalent to gaining or losing just
one second every 300 years. Exploiting micro-electro-mechanical systems (MEMS) chip
fabrication technology, the atomic clocks 18, 21, 24 have a volume of less than 0.1
cm
3 and consume only a few tenths of milliwatts of power, enabling the atomic clocks
18, 21, 24 to be used in solid state packages having a suitably small form factor.
[0018] A slave clock is a clock that is coordinated with a master clock, and the GPS slaving
unit 27 is used to slave at least one of the plurality of atomic clocks 18, 21, 24
to the GPS receiver 30 to achieve what is known as "clock coasting". Clock coasting
is a free-running operational timing mode in which continuous or periodic measurement
of clock error, i.e., of timing error, is not made, in contrast to tracking mode.
Operation in the coasting mode may be extended for a period of time by using clock-error
data or clock-correction data (obtained during a prior period of operation in the
tracking mode occurring at the clock processor 20) to estimate clock corrections for
the no-satellite situation.
[0019] Slave clock coordination is usually achieved by phase-locking the slave clock signal
to a signal received from the master clock, in this non-limiting example, a PPS signal.
The GPS slaving unit 27 is used for the phase-locking by noting the phase relative
to the master clock. To adjust for the transit time of the signal from the master
clock to the slave clock, the phase of the slave clock may be adjusted with respect
to the signal from the master clock so that both clocks are in phase. Thus, the time
markers of both clocks, at the output of the clocks, occur simultaneously.
[0020] Atomic clocks generally produce great short term precision but may suffer over long
periods with stability deficiencies. GPS clocks, on the other hand, have short term
stability deficiencies but are stable over longer periods. The distinct and complementary
natures of time derived by the atomic clock 30 and the time solution derived from
data received at the GPS receiver 12 assure greater accuracy of solutions of the RAIM
algorithms at the processor 27.
[0021] The GPS receiver 30 has a clock bias from GPS time as received. If a highly stable
clock reference is used, however, the GPS receiver 30 time could be based on the highly
stable clock without solving for a bias. "Clock coasting" requires an atomic clock
with superior long term stability, thereby combining the strengths of each time discerning
system to get a far more accurate and precise determination of system time.
[0022] The clock processor 20 facilitates coasting by accumulating errors for calculating
RAIM availability within a measurement period. The subsequent estimation for the next
calculations predict and constrain time values based on higher precision atomic clocks
18, 21, 24. RAIM can be synthesized to provide ephemeris for a missing satellite.
Synthesis is based upon an assumption that the user clock error "dynamics" are milder
than the vehicle dynamics, thus a clock processor 20 may be allowed to quit tracking
the clock error for a short period and determine integrity with only four satellites.
The length of time for which this may reasonably be done depends, of course, on the
user clock frequency stability.
[0023] The on board precise frequency standard is slave-locked to the GPS receiver 30 exploiting
a one PPS signal based on UTC. Within the clock processor 20, the on board time received
time stamps and the on board precision atomic clock form a closed loop system that
has the historical inaccuracies of the precision clock contained within the loop parameters,
allowing the instability of the clock to be effectively zeroed prior to a RAIM outage.
As a result, the clock processor 20 derives a signal through clock coasting that during
periods of RAIM outage is well determined and predictable due to the closed-loop hardware.
Where, as in one non-limiting embodiment one atomic clock 18 is used, the operation
of the clock 18 is compared to the time signal derived at the GPS receiver 30 to determine
the operable status of the clock 18.
[0024] In another non-limiting option, at least two calibrated and slaved atomic clocks
18 and 21 are exploited. The time outputs from the clocks 18 and 21 can be compared
during periods of coasting. Any inconsistency between two clocks triggers a failure
detection capability; thereby, to assure integrity of the clock signal, if one of
the clocks fails (and the failure is not a "common mode" failure, i.e. a failure that
kills off the accuracy of both clocks in the same way) a fault is indicated such that
the navigation solution is not trustworthy.
[0025] In an embodiment exploiting three or more clocks 18, 21, 24, an exclusion exploits
the output of two of the clocks that generally agree; for example, the second and
third atomic clocks 21 and 24. The remaining clock 18 is determined an outlier and
the output of the first atomic clock 18 is disregarded in developing a clock solution
at the clock processor 20. In the embodiment, an enunciator (not shown) might, optionally,
be used to signal the need for examining the trio of clocks 18, 21, 24 to determine
and correct the source of the fault.
[0026] Continued RAIM availability is facilitated by the availability of a precise frequency
standard within the navigational system 10. When the time solution derived from the
GPS receiver 30 is in error, the system processor 33 can isolate the offending satellite.
As a result, the system processor 33 will set an internal satellite health indicator
to "unhealthy" which causes the GPS receiver 30 to remove the satellite from the tracked
list. Furthermore, upon isolation, the system processor 33 recalculates the receiver
time and controls without the offending satellite. The system processor 33 will continue
to monitor the removed satellite and compare its derived time signal to the onboard
signal to determine when to set the internal satellite health indicator to healthy
again and include it in the tracked list.
[0027] In both embodiments, the time stamping is optionally based on Coordinated Universal
Time ("UTC") although any internally consistent time stamping convention will suitably
serve the ends of the invention. Ongoing time stamping of GPS and IMU data allows
the clock processor 20 to implement a closed loop system that measures the historical
inaccuracies of the atomic clocks 18, 21, 24. In operation, the clock processor 20
compares the atomic clocks' 18, 21, 24 time with the time solution from such GPS signals
as are received at the GPS receiver 30. With designated loop parameters, any instability
of the atomic clocks 18, 21, 24 is effectively zeroed prior to the RAIM outage. The
resulting clock coasting during the RAIM outage is well-determined and predictable
due to the closed-loop hardware.
[0028] A closed-loop multiple clock 30 system to reduce clock 30 errors in frequency, drift
and second order rate of change for RAIM calculation in the absence of over determination
(less than five satellites) of PVT and clock 30 errors and drift.
[0029] Referring to FIG. 2, a method 50 for executing a RAIM is illustrated The RAIM algorithm
is based upon an atomic clock signal received from at least one atomic clock. In some
embodiments, a plurality of clocks is disciplined with a process of clock coasting.
[0030] At a block 51, GPS ephemerides are received and identified as emanating from distinct
satellites. The number of distinct satellite ephemerides are identified. Conventional
RAIM algorithms rely upon ephemerides from six or more satellites. To obtain a 3-dimensional
position solution, at least 4 measurements are required. To detect a fault, at least
5 measurements are required, and to isolate and exclude a fault, at least 6 measurements
are required; however, more measurements are often needed depending on the satellite
geometry. Typically, there are 7 to 12 satellites in view.
[0031] If, at the block 51, six or more satellites were visible, then at a block 54, ephemerides
from the visible satellites are received to calculate RAIM based upon conventional
methods. With the RAIM solution, appropriate ephemerides are identified to derive
a time solution. With that time solution, at a block 60, the availability of GPS time
allows the slaving of one or more atomic clocks to a derived portion of a PVT solution
derived at a GPS receiver.
[0032] If, at the block 51, fewer than six satellites had been available, at a block 72,
the method 50 progresses to execute the RAIM algorithm using the adjusted or conditioned
atomic clock output as a substitute for the missing sixth satellite ephemeris. Upon
the execution of the RAIM algorithm using the conditioned atomic clock output, the
system responds by known means. By virtue of the closed-loop multiple clock system
to reduce clock errors in frequency, drift and second order rate of change for RAIM
calculation in the absence of over determination (less than five satellites) of PVT
and clock errors and drift, the RAIM solution will be accurate with one less satellite.
[0033] While the preferred embodiment of the invention has been illustrated and described,
as noted above, many changes can be made without departing from the spirit and scope
of the invention. Accordingly, the scope of the invention is not limited by the disclosure
of the preferred embodiment. Instead, the invention should be determined entirely
by reference to the claims that follow.
[0034] The embodiments of the invention in which an exclusive property or privilege is claimed
are defined as follows:
1. A method (50) for providing a substituted timing signal for a missing satellite ephemeris
in execution of a RAIM algorithm, the method comprising:
deriving a plurality of position, velocity, and time solutions from a GPS navigation
system derived from a plurality of satellite ephemerides (57);
receiving an atomic clock signal;
comparing (50) the atomic clock signal to the derived time solutions to derive a correction
factor;
adjusting (69) the atomic clock signal according to the correction factor to develop
an adjusted atomic clock signal; and
substituting (72) the adjusted atomic clock signal for a missing satellite ephemeris
to execute the RAIM algorithm.
2. The method of Claim 1, wherein the GPS navigation system (10) is an inertial GPS system.
3. The method of Claim 1, wherein the atomic clock (18, 21, 24) is a chip scale atomic
clock.
4. A GPS navigation system including a RAIM processor (33), the GPS navigation system
comprising:
a GPS receiver (30) for receiving satellite ephemerides from a plurality of GPS satellites,
the receiver configured to derive a plurality of position, velocity, and time solutions;
an atomic clock (18, 21, 24) producing a clock signal;
a clock follower (27) to compare time solutions from the GPS receiver to the clock
signals and deriving a correction factor to synthesize a corrected clock signal; and
a RAIM algorithm processor (33) to receive the satellite ephemerides and the time
solutions from the GPS receiver and the corrected clock signal to test the integrity
of each of the satellite ephemeris in the satellite ephemerides.
5. The system of Claim 4, wherein the GPS receiver includes an inertial measurement unit.
6. The system of Claim 4, wherein the ephemerides include at least one military GPS satellite
ephemeris.
7. The system of Claim 4, wherein the atomic clock (18, 21, 24) is a chip scale atomic
clock.
8. An apparatus for providing a substituted timing signal for a missing satellite ephemeris
in execution of a RAIM algorithm, the method comprising:
a GPS receiver (30) for deriving a plurality of position, velocity, and time solutions
from a GPS navigation system (10) derived from a plurality of satellite ephemerides;
an atomic clock (18, 21, 24) for generating an atomic clock signal;
a clock follower (27) for:
receiving an atomic clock signal;
comparing the atomic clock signal to the derived time solutions to derive a correction
factor;
adjusting the atomic clock signal according to the correction factor to develop an
adjusted atomic clock signal; and
a processor (33) for executing the RAIM algorithm based upon the adjusted atomic clock
signal substituted for a missing satellite ephemeris.
9. The system of Claim 8, wherein the atomic clock (18, 21, 24) is a chip scale atomic
clock.
10. The system of Claim 8, wherein at least one of the plurality of satellite ephemerides
is a PPS ephemeris.